Glyn Hopper, head of Sowerby County Primary School in Thirsk, North Yorkshire, was told of the problem by the pupils themselves, who discussed it at their school council meeting.

When she asked how many of the 200 junior school children had seen the show, nearly half admitted watching it.

The American-produced programme, broadcast on Channel 4 and Sky, features the outrageous antics of a group of foul-mouthed eight-year-olds.

It is aimed at adults, but has built up a cult following among adults and children alike.

Mrs Hopper, who says she has never seen South Park, said: "The children have a voice with their school council and I am proud of them for bringing this up.

"They had noticed that some children were copying the programme, for example, spoiling games because they thought it was cool. They also mentioned bad language was being used."

Mrs Hopper said the main aim of her newsletter for parents was to try to urge them to stop children watching programmes after the 9pm watershed.

'Valuable alert'

"I have had letters and phone calls from parents saying thank-you for the letter. Usually nobody responds to it," she said.

"The school does not have a problem. It's a lovely school and it is good that the children have a voice."

John Beyer, director of the National Viewers' and Listeners' Association, the independent watchdog group set up by Mary Whitehouse, said South Park was "vulgar".

"It shows how influential television can be and how children copy what they see," he said.

"What the teacher has done here, by alerting parents, is very valuable."

Responding to earlier criticism, Channel 4 has said that the show is broadcast "post-watershed" and is not aimed at children.

Mrs Hopper's plea to parents follows a similar request made last month by a public school in Cambridgeshire.

The King's School in Ely, wrote to parents telling them that the cartoon was "filth of a most unsavoury nature" and should not be watched by younger children.

Last year, an infant school in Surrey was renamed in an attempt to shed the unwanted image of the show.

South Park Infants in Reigate was rechristened The Orchards.